1. Field of the Invention
This invention relates to a refrigeration method that employs a process or processes, whereby a supercritical fluid is used in a vapor-compression thermodynamic cycle, and more particularly to a means of enhancing cooling capacity and efficiency.
2. Background
In conventional vapor-compression refrigeration cycles, heat is absorbed at a constant temperature by a fluid undergoing evaporation, vapor is then compressed to a higher pressure before giving up heat of evaporation, as well as work energy added during compression, in a condenser at a subcritical pressure, before ultimately decompressing through an expander and returning to the evaporator to pick up heat and begin the cycle anew. An alternative to this cycle is to compress the fluid to a supercritical state at a high enough pressure to ensure that it remains in a supercritical state as it releases heat to a cooling medium. In refrigeration cycles, the cooling medium is usually air, but it can be another fluid, such as seawater. Then, as the cooled working fluid is expanded, it returns to a subcritical state and condenses, after which it returns to the evaporator to absorb heat anew. Such a cycle is termed transcritical.
Throughout the history of vapor-compression refrigeration, stretching back over 150 years, subcritical cycles have been the norm. Early refrigeration devices based on carbon dioxide, ammonia or sulfur dioxide, worked in this way. Carbon dioxide was favored for commercial refrigeration in the early part of the Twentieth Century, but lost its importance to chlorofluorocarbon (CFC) refrigerants in the 1930s. These fluids were preferred because they reject heat at lower pressures, thus requiring smaller compressor capacity. They are also deemed non-toxic and safe. Within decades, the use of carbon dioxide as a refrigerant became uncommon.
In the early 1970s, however, the environmental risks posed by CFCs were realized. Theoretical estimations of ozone depletion were bolstered by observations of ozone xe2x80x9cholesxe2x80x9d over the Antarctic. The United Nations is leading a multinational movement to phase out the use of certain classes of CFCs, or to substitute them with grades that pose less ozone-depletion potential. Nevertheless, even the best substitutes present a long-term risk, and the search is on for a refrigerant that has no ozone-depletion potential. This has led to renewed interest in carbon dioxide.
This revived interest in carbon dioxide, however, comes with a general desire to achieve efficiencies at least as good as those experienced with CFC cycles. Consequently, most recent proposals of refrigeration devices based on carbon dioxide have called for operating under transcritical cycles.
The benefits of supercritical cooling have long been known. Operators of subcritical systems may have on occasion sought to coax more refrigeration capacity from their machines by raising compression pressure to cause more heat exhaustion to occur under supercritical conditions. If the temperature of the ambient cooling fluid rose significantly, as could be the case during hot summer days, this might have been necessary to maintain minimum refrigeration capability.
Brenan (U.S. Pat. No. 4,205,532) drew on this knowledge in patenting a heat pipe. This invention addresses the four basic components of a transcritical cycle: an accepter (or evaporator), a compressor, a rejecter that exhausts heat, and an expansion device. Brenan did not, however, offer a method for controlling the process, nor did he address methods to improve the thermodynamic efficiency of compression or expansion, the points at which the greatest extent of thermodynamic irreversibility take place. Providing control of compression and expansion is therefore needed to improve thermodynamic efficiency.
Lorentzen et at (U.S. Pat. No. 5,245,836) improved on Brenan by presenting a method of control that ensures sufficient mass flow to maintain supercritical conditions between the compressor outlet and expander inlet. The method involves controlling the pressure in the xe2x80x9chighxe2x80x9d side in or near the rejecter by throttling an expansion valve. Additionally, an accumulator is provided with the dual purpose of ensuring sufficient liquid in the system to maintain evaporation, even if the expander is throttled tightly, as well as to provide a means for separating compressor oil from the working fluid. The presence of compressor oil in the working fluid is a disadvantage, the means of separating the oil from the working fluid notwithstanding, because the heat transfer coefficient of the working fluid is decreased by the presence of the oil, thereby reducing overall efficiency.
Replacing a throttling valve with a turbine for fluid expansion has long been recognized. Williams (U.S. Pat. No. 4,170,116) supplemented a throttling valve with a turbine in series with the valve. Robinson and Groll, in Int. J. Refrig., 1998, elucidated the benefits of a turbine as the expander on its own, without a throttling valve. They demonstrated, by means of simulations, that a turbine can increase the Coefficient of Performance (COP) of a cycle over that which employs a conventional expansion valve. Furthermore, COP reaches an optimum depending on the heat rejection pressure. Means for controlling a practical process were not provided, however.
An important consideration in the application of a turbine is the method of recovering work energy from the turbine. Such methods are undeveloped in current practice. One possibility for work recovery, by which the turbine and the compressor are coupled, is commonplace in refrigeration systems based on air or nitrogen cycles. Transcritical refrigeration cycles, based on carbon dioxide, are emerging, especially in automotive air conditioning applications. The current state-of-the-art, however, has yet to implement all the means possible to achieve highest efficiency. Most significantly, little has been done to improve compressor efficiency. In automotive systems, efficiency is of secondary importance owing to the plentitude of power available from a vehicle""s powertrain.
Hazlebeck (U.S. Pat. No. 5,405,533) discloses a supercritical process that relies on thermosyphoning and thus omits the compressor completely. Such a system, however, is highly constrained in terms of the range of operating temperatures and portability. In order to build compact and efficient refrigeration devices, improvements to compressor efficiency and compactness are necessary.
It is therefore an object of the present invention to improve the efficiency of the transcritical vapor compression refrigeration cycles and to increase their capacity.
Another object of the present invention is to simplify the refrigeration process by avoiding the need for an accumulator that is otherwise employed for the purpose of providing a buffer for handling varying amounts of liquid-state working fluid in the system.
Another object of the present invention is to operate the refrigeration cycle with an oil-free working fluid and thereby simplify the refrigeration process by avoiding the need for an accumulator that is otherwise employed for the purpose of separating oil from the working fluid.
Another object of the present invention is to improve the efficiency of supercritical fluid refrigeration cycles over that of CFC refrigerants by operating the expansion and compression steps in such ways as to reduce thermodynamic irreversibitities. This includes the replacement of an expansion valve with a turbine for expansion, or the use of multi-stage compression, or a combination thereof.
Yet another object of this invention is to improve efficiency using a nontoxic and environmentally benign working fluid.
This invention relates to a method for refrigeration using a vapor compression cycle. The method includes the steps of:
(a) obtaining a natural, oil-free refrigerant;
(b) compressing the said refrigerant;
(c) transferring heat from the refrigerant to an external environment through one or more heat exchangers;
(d) expanding the said refrigerant isentropically;
(e) transferring heat from an external environment to the refrigerant through one or more heat exchangers;
(f) connecting the above mentioned components in a closed loop;
(g) circulating said refrigerant in said loop through a cycle involving supercritical high pressure and subcritical low pressure conditions;
(h) controlling the mass flow rate; and
(i) refrigerating the external environment.
The said refrigerant is non-toxic and environmentally benign. The said refrigerant is selected from a group consisting of carbon dioxide, water, a hydrocarbon or a combination thereof. The said refrigerant can be compressed by a compressor, which may be of a reciprocating or centrifugal type. After giving up heat in a heat exchanger, the said refrigerant then is expanded in a turbine, which may be of an impulse or reaction type. The inlet mass flow to the compressor is varied by changing the compression stroke, changing the final compression volume or changing the speed of the compressor drive, wherein the efficiency of the turbine is more than 60%. The turbine produces useful work and may be energetically coupled with the compressor to recover energy.
At least 30% of the total volume of said refrigerant, operating in a vapor compression cycle according to the method described herein, occupies the low pressure side of the system. At least 15% of the total mass of said refrigerant, operating in a vapor compression cycle according to the method described herein, occupies the low pressure side of the system.
In further aspects of this invention, said refrigerant is expanded isentropically, thereby increasing capacity and efficiency. One or more intercoolers transfer useful heat from the high pressure side and to the low pressure side. One or more separators are used to separate gas and liquid. A combination of intercoolers and separators are used to transfer useful work from the high pressure side to the low pressure side and to separate gas and liquid. The oil-free refrigerant increases the efficiency of the cycle. Control of the mass flow rate is accomplished through control of compressor. The mass flow rate is controlled by one or more of the following means: varying the inlet mass flow to the compressor, changing the compression stroke, changing the final compression volume or changing the speed of the compressor drive.
This invention also relates to an apparatus for refrigeration using a vapor compression cycle. The apparatus consists of:
(a) a compressor to compress a natural, oil-free refrigerant;
(b) one or more heat exchangers for transferring heat from the refrigerant to an external environment;
(c) a turbine for isentropic expansion of the refrigerant;
(d) one or more heat exchangers for transferring heat from the refrigerant to an external environment;
(e) a closed loop for a fluid connection of the above mentioned components;
(f) means for circulating said refrigerant in said loop through a cycle involving
(g) supercritical high pressure and subcritical low pressure conditions; and means to control the mass flow rate;
wherein, the components of the apparatus are of the type previously described so as to perform in accordance with the aforementioned methods of this invention.